U.S. nuclear energy facilities generated electricity at a record high level of reliability in 2015, preliminary estimates show.

Ninety-nine nuclear power plants operating in 30 states posted an estimated average capacity factor of 91.9 percent, based on preliminary 2015 data compiled by the Nuclear Energy Institute. That surpasses the industry’s prior record set in 2007 by one-tenth of a percentage point. Capacity factor measures the total electricity generated as a percentage of potential generation for the entire year.

“The 2015 data confirm yet again what a tremendous asset nuclear energy technology is,” said NEI Chief Nuclear Officer Anthony Pietrangelo. “U.S. nuclear power plants continue to operate at exceptional levels of safety and reliability, while generating affordable electricity that consumers and our economy rely on. This is due to the hard work and dedication of the highly skilled men and women who work at these facilities and the reactor vendors and suppliers who provide support services.”

Actual electricity production from nuclear energy facilities last year was the fifth-highest ever, at an estimated 797.9 billion kilowatt-hours (kWh). The amount of electricity generated in 2015 exceeded the amount produced in 2014, even though one less plant operated due to the 2014 closing of Vermont Yankee. This record efficiency is even more important as states strive to meet new carbon-reduction targets for the electric sector.

The industry’s record high electricity generation came in 2010, when the 104 reactors then operating produced 806.9 billion kWh of electricity while posting an industry average capacity factor of 90.9 percent.

Nuclear energy facilities have annually produced about one-fifth of America’s electricity supplies for the past two decades, even as total electricity demand has increased significantly. Because of their electric sector-leading capacity factors, they have done so even though nuclear power plants constitute only about 10 percent of the nation’s installed electric generating capacity.

During periods of extreme weather, the value of nuclear energy facilities is even greater. With December 2015 being both the hottest and wettest in the continental United States since record-keeping began, nuclear plants withstood historic flooding that overwhelmed the central states from late December to mid-January of this year. Affected nuclear facilities included Fort Calhoun and Cooper Nuclear Station in Nebraska, Callaway in Missouri, Arkansas Nuclear One, Grand Gulf in Mississippi, and River Bend and Waterford in Louisiana. In all cases, the plants’ flood protection measures enabled reliable operation throughout the event.